1. Field of the Invention
The present invention relates to an OLED panel having an organic OLED (OLED: organic light emitting device) formed on a substrate, sealed between the substrate and a cover material. Moreover, it relates to an OLED module having an IC, or the like including a controller packaged on the OLED panel. In this specification, both the OLED panel and the OLED module are referred to as the light emitting device. Furthermore, the present invention relates to an electronic appliance using the light emitting device.
2. Description of the Related Art
The OLED itself emits a light so as to provide a high visibility so that backlighting necessary for a liquid crystal display device (LCD) is not required, and thus it is suitable for providing a thin shape as well as the view angle is not limited. Therefore, recently, a light emitting device using an OLED attracts the attention as the display device for replacing the CRT and the LCD.
The OLED has a layer including an organic compound (organic light emitting material) for obtaining a luminescence (electroluminescence) to be generated by the application of the electric field (hereinafter referred to as an organic light emitting layer), an anode layer, and a cathode layer The luminescence in an organic compound include the light emission (fluorescence) at the time of returning from the singlet excitation state to the ground state, and the light emission (phosphorescence) at the time of returning from the triplet excitation state to the ground state. In the light emitting device of the present invention, either one of the above-mentioned light emissions may be used, or both of the light emissions may be used as well.
In this specification, all the layers provided between the anode and the cathode of the OLED are defined to be an organic light emitting layer. Specifically, the organic light emitting layers include a light emitting layer, a positive hole injecting layer, an electron injecting layer, a positive hole transporting layer, an electron transporting layer, or the like. Basically, the OLED has a structure with the anode, the light emitting layer, and the cathode successively. In addition to the structure, it may have a structure with the anode, the positive hole injecting layer, the light emitting layer, and the cathode, or a structure with the anode, the positive hole injecting layer, the light emitting layer, the electron transporting layer, the cathode, or the like in this order.
It has been problematic at the time of putting the light emitting device into practice that the luminance of the OLED is lowered according to deterioration of the organic light emitting material.
The organic light emitting material is weak with respect to the moisture content, the oxygen, the light, and the heat so that deterioration is promoted thereby. Specifically, the deterioration rate depends on the structure of the device for driving the light emitting device, the characteristics of the organic light emitting material, the material of the electrode, the condition in the production step, the driving method for the light emitting device, or the like.
Even in the case the voltage applied on the organic light emitting layer is constant, if the organic light emitting layer is deteriorated, the luminance of the OLED is lowered so that the displayed image is not sharp. In this specification, a voltage applied to the organic light emitting layer from a pair of electrodes is defined to be an OLED driving voltage (Vel).
Moreover, in a color display method using three kinds of the OLEDs corresponding to R (red), G (green), and B (blue), the organic light emitting material comprising the organic light emitting layer differs depending on the color corresponding to the OLED. Therefore, the organic light emitting layers may deteriorate by different rates according to the corresponding color. In this case, the luminance of the OLED differs per each color so that an image having a desired color cannot be displayed on the light emitting device.
Furthermore, the temperature of the organic light emitting layer depends on the heat of the external atmosphere, temperature of the heat generated by the OLED panel itself, or the like. In general, the OLED has the flowing current value changed according to the temperature. FIG. 26 shows the change of the voltage current characteristics of the OLED with the temperature of the organic light emitting layer changed. In the case the voltage is constant, if the temperature of the organic light emitting layer is raised, the OLED driving current is enlarged. Since the OLED driving current and the luminance of the OLED have a proportional relationship, the higher the OLED driving current is, the higher the luminance of the OLED is. Accordingly, since the luminance of the OLED is changed depending on the temperature of the organic light emitting layer, it is difficult to display a desired gradient so that the current consumption of the light emitting device is enlarged according to the temperature rise.
Moreover, in general, since the degree of the change of the OLED driving current by the temperature change differs depending on the kind of the organic light emitting material, the luminance of the OLEDs of each color may change independently by the temperature in the color display. In the case the luminance of each color is not balanced, desired color cannot be displayed.
Accordingly, in view of the above-mentioned circumstances, an object of the present invention is to provide a light emitting device capable of obtaining a constant luminance regardless of the organic light emitting layer deterioration or the temperature change, and further capable of providing a desired color display.
The present inventor has paid attention to the fact that the OLED luminance decline by the deterioration is smaller in the latter case in comparison between the light emission with the OLED driving voltage maintained constantly (the former case) and the light emission with the current flowing in the OLED maintained constantly (the latter case). In this specification, the current flowing in the OLED is referred to as the OLED driving current (Iel). Then, it is considered that the change of the OLED luminance by the OLED deterioration can be prevented by controlling the OLED luminance not by the voltage but by the current.
Specifically, in the present invention, a current mirror circuit comprising a transistor is provided in each pixel so that the OLED driving current is controlled using the current mirror circuit. Then, the first transistor and the second transistor of the current mirror circuit are connected such that the drain currents thereof can be maintained at the substantially equal value regardless of the load resistance value.
In this specification, a size of a current is an absolute value of a current.
The first transistor has the drain current I1 thereof controlled by a signal line driving circuit Since the size of the drain current I1 is provided always equal to the size of the drain current I2 of the second transistor regardless of the load resistance value, as a result, the drain current I2 of the second transistor is controlled by the signal line driving circuit.
The second transistor is connected such that the drain current I2 thereof flows into the OLED. Therefore, the value of the OLED driving current flowing in the OLED is controlled not by the load resistance but by the signal driving circuit. In other words, the OLED driving current can be controlled at a desired value regardless of the difference of the transistor characteristics, deterioration of the OLED, or the like.
In the present invention, according to the above-mentioned configuration, decline of the luminance of the OLED can be restrained even in the case the organic light emitting layer is deteriorated, and as a result, a sharp image can be displayed. Moreover, in the case of a color display light emitting device using the OLED corresponding to each color, even in the case the organic light emitting layers of the OLED are deteriorated by different rates per each corresponding color, a desired color can be displayed by preventing deterioration of the balance of the luminance among the colors.
Furthermore, even in the case the temperature of the organic light emitting layer is influenced by the external atmosphere temperature, the heat generated by the OLED panel itself, or the like, the OLED driving current can be controlled at a desired value. Therefore, since the OLED driving current and the luminance of the OLED are proportional, change of the luminance of the OLED can be restrained, and further, increase of the current consumption according to the temperature rise can be prevented. Moreover, in the case of a color display light emitting device, since change of the luminance of the OLED of each color can be restrained regardless of the temperature change, deterioration of the balance of the luminance among the colors can be prevented so that a desired color can be displayed.
Furthermore, in general, since the degree of the change of the OLED driving current in the temperature change differs depending on the kind of the organic light emitting material, the luminance of the OLED of each color can be changed independently in the color display. However, according to the light emitting device of the present invention, since a desired luminance can be obtained regardless of the temperature change, deterioration of the balance of the luminance among the colors can be prevented so that a desired color can be displayed.
Moreover, in an ordinary light emitting device, since the wiring for supplying the current to each pixel itself has a resistance, the potential thereof is slightly lowered depending on the length of the wiring. The potential decline differs largely depending also on the image to be displayed. In particular, in the case the ratio of pixels of a high gradient number is high in a plurality of pixels having the current supplied from the same wiring, the current flowing in the wiring is increased so that the potential decline becomes conspicuous. In the case the potential is lowered, since the voltage applied on the OLED Of each pixel becomes small, the current supplied to each pixel becomes small. Therefore, even in the case a constant gradient is to be displayed in a predetermined pixel, if the gradient number of the other pixel having the current supplied from the same wiring is changed, the current supplied to the predetermined pixel is changed thereby so that the gradient number is changed as a result. However, according to the light emitting device of the present invention, since the OLED current can be corrected by obtaining the measured value and the reference value for each image to be displayed, a desired gradient number can be displayed by the correction even in the case the image to be displayed is changed.
In the light emitting device of the present invention, the transistor to be used for the pixel may be a transistor using a single crystal silicon, or a thin film transistor using a polycrystalline silicon or an amorphous silicon.